[PATCH 06/14] misc: Add support for nvmem cells

[Sean Anderson]

On 2/26/22 1:36 PM, Simon Glass wrote:
> Hi Sean,
> 
> On Mon, 7 Feb 2022 at 16:42, Sean Anderson <sean.anderson at seco.com> wrote:
>>
>> This adds support for "nvmem cells" as seen in Linux. The nvmem device
>> class in Linux is used for various assorted ROMs and EEPROMs. In this
>> sense, it is similar to UCLASS_MISC, but also includes
>> UCLASS_I2C_EEPROM, UCLASS_RTC, and UCLASS_MTD. While nvmem devices can
>> be accessed directly, they are most often used by reading/writing
>> contiguous values called "cells". Cells typically hold information like
>> calibration, versions, or configuration (such as mac addresses).
>>
>> nvmem devices can specify "cells" in their device tree:
>>
>>         qfprom: eeprom at 700000 {
>>                 #address-cells = <1>;
>>                 #size-cells = <1>;
>>                 reg = <0x00700000 0x100000>;
>>
>>                 /* ... */
>>
>>                 tsens_calibration: calib at 404 {
>>                         reg = <0x404 0x10>;
>>                 };
>>         };
>>
>> which can then be referenced like:
>>
>>         tsens {
>>                 /* ... */
>>                 nvmem-cells = <&tsens_calibration>;
>>                 nvmem-cell-names = "calibration";
>>         };
>>
>> The tsens driver could then read the calibration value like:
>>
>>         struct nvmem_cell cal_cell;
>>         u8 cal[16];
>>         nvmem_cell_get_by_name(dev, "calibration", &cal_cell);
>>         nvmem_cell_read(&cal_cell, cal, sizeof(cal));
>>
>> Because nvmem devices are not all of the same uclass, supported uclasses
>> must register a nvmem_interface struct. This allows CONFIG_NVMEM to be
>> enabled without depending on specific uclasses. At the moment,
>> nvmem_interface is very bare-bones, and assumes that no initialization
>> is necessary. However, this could be amended in the future.
>>
>> Although I2C_EEPROM and MISC are quite similar (and could likely be
>> unified), they present different read/write function signatures. To
>> abstract over this, NVMEM uses the same read/write signature as Linux.
>> In particular, short read/writes are not allowed, which is allowed by
>> MISC.
>>
>> The functionality implemented by nvmem cells is very similar to that
>> provided by i2c_eeprom_partition. "fixed-partition"s for eeproms does
>> not seem to have made its way into Linux or into any device tree other
>> than sandbox. It is possible that with the introduction of this API it
>> would be possible to remove it.
>>
>> Signed-off-by: Sean Anderson <sean.anderson at seco.com>
>> ---
>>
>>  MAINTAINERS           |   7 ++
>>  doc/api/index.rst     |   1 +
>>  doc/api/nvmem.rst     |   7 ++
>>  drivers/misc/Kconfig  |  16 ++++
>>  drivers/misc/Makefile |   1 +
>>  drivers/misc/nvmem.c  | 109 +++++++++++++++++++++++++
>>  include/nvmem.h       | 185 ++++++++++++++++++++++++++++++++++++++++++
>>  7 files changed, 326 insertions(+)
>>  create mode 100644 doc/api/nvmem.rst
>>  create mode 100644 drivers/misc/nvmem.c
>>  create mode 100644 include/nvmem.h
> 
> Here I think it would be better to add a new uclass so that drivers
> which support it can add a child device in that uclass. This is done
> in lots of places in U-Boot.

I'm not sure exactly what you have in mind. The issue is that there are at
least 6 uclasses which I would like to support:

- UCLASS_MISC
- UCLASS_I2C_EEPROM
- UCLASS_RTC
- UCLASS_MTD
- UCLASS_FUSE (doesn't exist yet, but probably should)
- Possibly UCLASS_PMIC

Most of these uclasses have existing interfaces which expose an NVMEM-like API,
in addition to other uclass-specific functionality. Instead of having an
additional API which drivers must implement, I would like to leverage these
existing APIs to make adding NVMEM support as painless as possible. NVMEM is
more of a "meta-uclass" which allows us to leverage existing read/write
functions in uclasses. If any additional devices are to be created, they need
to be created by the nvmem subsystem, or by the supported uclasses, rather than
in drivers.

Now, there are some instances where creating a new child device might be the
best approach. For example, you might have some OTP memory in an unrelated
device. In that case, creating a child device (of UCLASS_MISC, or UCLASS_FUSE
if that gets created) is the right course of action.

--Sean